Eutetramorium

Bolton, Barry & Fisher, Brian, 2014, The Madagascan endemic myrmicine ants related to Eutetramorium (Hymenoptera: Formicidae): taxonomy of the genera Eutetramorium Emery, Malagidris nom. n., Myrmisaraka gen. n., Royidris gen. n., and Vitsika gen. n .., Zootaxa 3791 (1), pp. 1-99 : 6-8

The Eutetramorium genus group Diagnosis of worker

Members of the subfamily Myrmicinae with the following characters in combination.

1 Mandible triangular, with 5-11 teeth; trulleum closed; masticatory margin longer than basal margin.

2 Palp formula predominantly 5,3, uncommonly 4,3, or 4,2, the palpomeres unspecialised.

3 Stipes with transverse crest varying from strongly present to absent.

4 Clypeus usually with a narrow anterior apron; without a differentiated median carina; median portion of clypeus usually longitudinally costulate or rugulose, not bicarinate but its lateral margins may be enhanced; anterior margin with or without an unpaired median seta.

5 Clypeus with median portion moderately broadly inserted between frontal lobes; in full-face view width of clypeus between the frontal lobes, at their point of maximum divergence, is at least 1.25 times greater than the maximum width of one of the lobes, and is 0.40-0.48 times the width across both lobes at their point of maximum divergence.

6 Torulus with upper lobe visible or concealed; when visible the torulus lobe projects beyond the lateral margin of the frontal lobe at or just behind the widest point of the lobe.

7 Frontal carinae absent or present behind the frontal lobes; antennal scrobes absent, or present above the eye.

8 Antenna 12-segmented, with an apical club of 3-5 segments.

9 Eyes present, in front of to slightly behind the midlength of the head capsule in full-face view.

10 Promesonotal suture absent from dorsum of mesosoma.

11 Propodeum unarmed, bidentate or bispinose, the dorsum frequently long; propodeal lobes rounded.

12 Propodeal spiracle does not abut the margin of the declivity.

13 Mesothoracic anteroventral transverse trench present on each side of a median crest.

14 Metasternal process present only in large Eutetramorium ; otherwise absent, but usually a pair of carinae present that diverge posteriorly from each side of the metasternal pit and frequently reach the inner margins of the metacoxal foramina; these carinae are usually highest on each side of the metasternal pit and sometimes visible as a low peak in profile.

15 Propodeal foramen with its anterior margin extending anterior to the posterior margins of the metacoxal foramina.

16 Procoxae larger than mesocoxae or metacoxae.

17 Tibial spurs: mesotibia 0 or 1, metatibia 0 or 1; simple when present, sometimes very slender and almost indistinguishable from the setae at the tibial apices.

18 Petiole nodiform and with an anterior peduncle, the spiracle usually close to the midlength of the peduncle, never at the node, never abutting the anterior articulation.

19 Subpetiolar process present anteroventrally on peduncle, appearing in profile as an angle or a small tooth, in ventral view usually as a transverse ridge or crest.

20 Peduncle of petiole anteroventrally with a roughly U-shaped articulatory area that narrows posteriorly to the subpetiolar process, which forms a posterior margin. The articulatory area is marginate on each side, and is bounded anteriorly by a transverse strip of cuticle to which the intersegmental membrane of the propodeal foramen is attached.

21 Stridulitrum present on pretergite of first gastral segment (abdominal segment 4).

22 Tergite of abdominal segment 4 does not overlap the sternite on the ventral surface of the gaster.

23 Gastral shoulders weakly present to absent.

24 Sting well developed and functional, simple, without an apical lamellate appendage, never spatulate, never thread-like.

25 Main pilosity of head and body simple, frequently, but not universally, absent from propodeal dorsum.

Comments on Eutetramorium group worker characters

It is not yet possible to indicate any unequivocal morphological apomorphies of this endemic Madagascan group. The subfamily Myrmicinae is huge, with frequent convergence and parallelism among characters so that analogues of each appear to have been developed independently several to many times. As a result, the best that can be attempted here is an inclusive diagnosis. However, the group is retrieved as an unambiguous monophyletic clade by molecular techniques (Prof. Philip S. Ward, pers. com., currently unpublished). The numbers below refer to the character numbers listed above.

2 Palp formula 5,3 is universal in the group except in Eutetramorium itself. There, two species have PF 4,3, and one has PF 4,2. A palp formula of 5,3 is common in Myrmicinae , occurring in 33 of 136 genera (Bolton, 2003). Palp formula 4,3 is slightly less commonly encountered in the subfamily (28 genera out of 136), but PF 4,2 is rare, being predominant only in the Neotropical Attini and in a few species scattered through six other genera.

3 The stipital crest is present in all genera of the group except Royidris . In Eutetramorium and Vitsika it is coarse, and the area of the stipes distal of the crest is depressed and concave. In Royidris the crest is very reduced, functionally absent, at best visible as a weak line across the stipes. In the other genera the crest is fine and curves across the stipes, usually just distal of its midlength, and is often more strongly developed at the median border of the stipes (adjacent to the labium), tending to decrease in definition as the lateral stipital border is approached.

4 The clypeal apron is very reduced only in Malagidris sofina . An unpaired seta at the midpoint of the anterior clypeal margin is characteristic of Malagadris , Royidris and Vitsika ; a pair of short, fine setae that straddle the midpoint occurs in Myrmisaraka , where the margin is also notched at the midpoint, and in Eutetramorium , where the paired setae are on each side of a median tooth or prominence.

6 In most Vistika species, and in Eutetramorium , the torulus lobe is concealed by the broad frontal lobe on each side.

7 Elongate frontal carinae and antennal scrobes are developed only in Vitsika , elsewhere the frontal carinae are represented only by the frontal lobes.

8 The antennal club is usually 3-segmented. It is 4-segmented in the Royidris notorthotenes group, and weakly 5-segmented only in Myrmisaraka producta .

13 On the ventral mesosoma there is a transverse depression or trench, U-shaped in section, on each side of a median longitudinal ridge of cuticle, located at the anterior end of the mesothorax just posterior to the procoxal cavities. The trench is commonly developed in myrmicines, but in numerous genera and groups of genera it is narrow, reduced or poorly defined (e.g. Atopomyrmex , Gauromyrmex , Myrmicaria , Ocymyrmex , the Leptothorax and Temnothorax genus groups), and in some it is extremely reduced or absent (e.g. Tetramorium , Cephalotes , Cataulacus ). The various developmental states of this structure, and their phylogenetic significance, await a proper investigation.

14 In Eutetramorium mocquerysi a large metasternal process is present, and there is also a bidentate mesosternal process.

17 In almost all species of the group a single mesotibial and metatibial spur is present, and obviously distinguishable from the setae at the tibial apices. However, in smaller species of Vitsika and Royidris reduction of the spurs is extreme, and they become indistinguishable from the apical setae under light microscopy.

20 The comparative morphology of the propodeal-petiolar articulation has never been investigated, but shows considerable variation through the Myrmicinae . The structure as described above for the Eutetramorium group appears most closely paralleled in Temnothorax and Huberia .

22 and 23 In numerous myrmicines, for example the solenopsidines and pheidolines (among others, see Bolton, 2003, for distribution) the fourth abdominal (first gastral) tergite strongly overlaps the sternite ventrally, so much so that in ventral view the tergite is extensively represented on the ventral surface. The tergite overlaps the sternite from each side and the inner margins of the tergite form a pair of convex longitudinal arcs in ventral view. Because of this the visible sternite appears narrowest at about its midlength and broadens posteriorly as the tergal arcs diverge. Anteroventrally, as the convex arcs of the tergal margins diverge, before suddenly curving in to the articulation of postpetiole and first gastral segment, the sternite tends to project forward beyond the margins of the tergite, to form a pair of shoulders that project anteriorly, one on each side of the articulation. These anterior projections of the sternite are frequently visible in dorsal view and constitute the gastral shoulders. The alternative, without the developments just described, occurs in the Eutetramorium group, and in many other groups. In ventral view the fourth abdominal (first gastral) tergite margins the sternite laterally, but does not extensively overlap onto the ventral surface. The inner margins of the tergite tend to be convex to more or less straight, and usually diverge posteriorly. Thus the visible sternite does not appear narrowest at about its midlength, but instead usually broadens posteriorly. Anteroventrally, the tergal margins converge evenly to the articulation of the postpetiole with the first gastral segment, and the sternite on each side does not project forward beyond the margins of the tergite. As a result there are no shoulders that project anteriorly on each side.

The myrmicine genus that morphologically corresponds most closely to the Eutetramorium group definition above appears to be Huberia Forel, a strange little genus of only two species that is confined to New Zealand (Brown, 1958; Don, 2007). Its type-species, H. striata (F. Smith) , largely matches the definition of the Eutetramorium group, but differs significantly as the upper lobe of the torulus projects anterior to the widest point of the frontal lobe, there is no clypeal apron, a large metasternal process is present (as is a mesosternal process), the antenna has 11 segments, the vestigial track of the promesonotal suture is retained across the dorsum of the mesosoma, and the propodeum is relatively short.